Could you please help? When we purchased the HP Z38C monitor for dual use with our iMac, we were told by the manufacturer that the correct connection wire would be included to connect a 2017 iMac using MacOS 10.15.4 to the HP z38c. We have tried each configuration possible without the settings being able to detect the monitor.

Ensure that the monitor has it's own power cable attached. Disconnect the USB-C cable from the iMac. Turn the monitor on and it should turn on and indicate no signal and perhaps go into power save mode. Open up System Preferences->Displays and plug the USB-C cable to the iMac and take note of any flickers on the iMac and any transitions on the monitor. Also note on the Displays panel if a tab called Arrangement appears or not.

Ensure that the monitor has it's own power cable attached. Disconnect the USB-C cable from the iMac. Turn the monitor on and it should turn on and indicate no signal and perhaps go into power save mode. Open up System Preferences->Displays and plug the USB-C cable to the iMac and take note of any flickers on the iMac and any transitions on the monitor. Also note on the Displays panel if a tab called Arrangement appears or not.

I believe I followed your instructions correctly.

There were no flickers on the iMac and no transitions on the monitor.

The Arrangement tab has not yet appeared either.

I also tried holding down the OPTIONS key to make the DETECT DISPLAYS button appear, but when clicked, nothing happens.

If you have another machine (or a friends machine) that has a USB-C connector to confirm that both the monitor and the cable are OK. It's just a matter of isolating things now. I know when I wanted to connect my 2009 iMac to an external display using the miniDB->DVI cable, I had to do a lot of research to find the right cable since a lot of people complained about connection/sleep issues with a lot of cables.

If you have another machine (or a friends machine) that has a USB-C connector to confirm that both the monitor and the cable are OK. It's just a matter of isolating things now. I know when I wanted to connect my 2009 iMac to an external display using the miniDB->DVI cable, I had to do a lot of research to find the right cable since a lot of people complained about connection/sleep issues with a lot of cables.

If you have another machine (or a friends machine) that has a USB-C connector to confirm that both the monitor and the cable are OK. It's just a matter of isolating things now. I know when I wanted to connect my 2009 iMac to an external display using the miniDB->DVI cable, I had to do a lot of research to find the right cable since a lot of people complained about connection/sleep issues with a lot of cables.